This invention may be applied to locking hubs (hub locks) for vehicles which convert between two-wheel drive and four-wheel drive operation. Typically such vehicles have two wheels, (e.g., the back wheels) that are permanently interconnected to the vehicle's drive line. The drive line also extends to the front two wheels but clutch mechanisms are provided for each of the front wheels that enable disconnecting and connecting each of the front wheels to a front drive axle of the drive line. Various types of control mechanisms are employed to actuate the clutch mechanisms. Typically the control mechanism of each wheel moves a clutch ring that will either become engaged with both the wheel hub and the drive axle or the clutch ring will be moved to be engaged with only one member, either the wheel hub or the drive axle. For purposes of discussion, the clutch ring will hereafter be referred to as permanently engaged with the wheel hub and is movable into engagement and out of engagement with the drive axle. The clutch rings of each front wheel when moved to be engaged with both the wheel hub and the drive axle provides for four wheel drive mode, that is the drive axles of the vehicle will drive all four wheels. The clutch rings of each wheel when moved to a position where they engage only the wheel hub disconnects the wheel hub from the drive axle and provides for two wheel drive mode.
The control mechanism can be manually actuated or automatically actuated. The automatic controls can be mechanically actuated (typically cam actuated) or pressure actuated (such as by positive or negative air pressure).
Pressure actuation has a number of advantages but also some serious disadvantages. The actuating mechanism is housed within the wheel hub. A piston is moved back and forth by changing the pressure relationship as between the two sides of the piston. The air pressure on one side has to be variable and, thus, requires an enclosure to create an air tight chamber that is connected to an air pressure source. The air pressure source is readily available on engine powered vehicles (e.g., negative air pressure is drawn to the manifold) but the air chamber encompasses moving parts that requires sealing. Several different seals are required for creating the air tight chamber within the wheel hub.
Typically the piston is biased to move in one direction to a first position by a spring. The application of air pressure opposes the spring pressure to move the piston in the opposite direction to a second position. The piston is maintained in the second position only so long as the air pressure is maintained at a level to overcome the spring. A leak in any one of the several seals can produce malfunction of the clutch. In addition to the possibility of malfunction due to seal leakage, the piston will not be maintained in the second position when the pressure is lost, such as by stopping the vehicle engine. The mechanism will have to be actuated again once the engine is restarted to move the piston to the second position.